1. Field of the Invention
The present invention relates to a positioning method and apparatus suitable for an exposure apparatus employed in a lithography process of manufacturing semiconductor elements and liquid crystal devices.
2. Related Background Art
In recent years, in the lithography process of manufacturing the semiconductor elements, a reduction projection type exposure apparatus of a step-and-repeat system, a so-called stepper, has been often employed as an apparatus for transferring patterns formed on a mask or a reticle onto a photosensitive substrate (a wafer or a glass plate having its surface on which a photoresist is applied with a high resolving power. In this type of stepper is provided with an alignment sensor of a TTL (Through The Lens) system which is disclosed in, e.g., U.S. Pat. No. 5,151,750. The alignment sensor is intended to precisely align a projection image of the reticle pattern with each of a multiplicity of circuit patterns formed on the wafer. This alignment sensor is a combined version of an alignment sensor (laser Step Alignment; LSA system) disclosed in, e.g., U.S. Pat. No. 4,677,301 and an alignment sensor (Laser Interferometric Alignment; LIA system) disclosed in, e.g., U.S. Pat. No. 4,710,026, whereby the optical members are shared at the maximum. Herein, the LSA system irradiates on-wafer alignment marks (diffraction grating marks) with elongate band-like spot beams. Beams of diffraction light generated from the marks are photoelectrically detected. On the other hand, the LIA system irradiates the diffraction grating marks with two laser beams coming from two directions different from each other, thereby forming one-dimensional interference fringes. There are photoelectrically detected beams of interference light of the diffraction beams generated from the marks in the same direction.
At present, a dominant alignment system of the stepper is an enhanced global alignment system (EGA) disclosed in, e.g., U.S. Pat. Nos. 4,780,617 and 4,833,621. The following is an explanation of the EGA system. At least three, e.g., seven shot areas located in the vicinity of the outer periphery of the wafer are selected from a multiplicity of circuit pattern areas (shot areas) on the wafer. This selection is done in advance of effecting an overlay exposure on a single piece of wafer. Further, two (X- and Y-directional) alignment marks (wafer marks) attached to each of these selected shot areas are detected by an alignment sensor. Coordinate positions of each mark are measured (sample alignment). Thereafter, totally six error parameters relative to array characteristics of the shot areas on the wafer are determined based on those mark positions (measured values) and design values by statistic calculations (least squares method). The error parameters are, to be specific, offsets (X- and Y-directions) of the central position of the wafer, degrees of expansion/contraction (X- and Y-directions) of the wafer, a residual rotational quantity of the wafer and a perpendicularity of the wafer stage (or perpendicularity of the shot array). Then, design coordinate values are corrected with respect to all the shot areas on the wafer on the basis of values of the determined parameters. The wafer stage is stepped according to the corrected coordinate values (calculated values). As a result, a projection image of the reticle pattern is overlaid on each of the multiplicity of shot areas on the wafer, thereby performing the exposure.
By the way, when performing the exposure with the reticle pattern superposed on the wafer shot area, the projection image of the reticle pattern and the shot area deviate in terms of positions in the X- and Y-directions. The positional deviations are caused due to changes both in reticle position and in baseline and also variations both in projection magnification of a projection optical system and in distortion. This conduces to such a problem that a sufficient overlay (alignment) accuracy can not be obtained. Under such circumstances, according to the prior arts, the overlay exposure (trial burning) of the reticle patterns (vernier patterns, etc.) on a pilot wafer is conducted. The alignment sensor or an inspection device for an exclusive use detects a deviation quantity between two (a main scale pattern and a vernier pattern) resist images formed in the wafer by development processing. When actually effecting the overlay exposure, the above-mentioned deviation quantity is given as an offset to the measured result of the alignment sensor. The projection image of the reticle pattern can be thereby superposed precisely on the shot area.
The above-mentioned method, however, requires the pilot wafer for the exclusive use of the measurement. Besides, a problem arises, wherein a measuring time increases depending on the development processing. For this reason, as disclosed in, e.g., U.S. Pat. No. 4,741,622, there is proposed a method of utilizing the latent images formed on a resist layer by irradiating the reticle marks with the exposure light. According to the method disclosed in the forgoing publication, the reticle pattern is aligned with one shot area on the wafer by use of the alignment sensor. Thereafter, the reticle mark is irradiated with the exposure light in advance of a main exposure, thereby forming its latent image on the resist layer. Then, the identical alignment sensor simultaneously detects latent images of wafer marks (base marks) already formed concomitantly with the shot area and of the reticle marks formed in the vicinities of the wafer marks. Positional deviation quantities therebetween are thus obtained. Further, the reticle and the wafer are relatively moved in accordance with these positional deviation quantities. That is, a realignment is executed. After this realignment, the main exposure is started, and the projection image of the reticle pattern can be precisely overlaid and exposed on the on-wafer shot area. Note that the alignment method utilizing the latent images is also disclosed in, e.g., U.S. Pat. Nos. 4,640,619, 5,124,927, 5,148,214, 5,140,366 and 5,262,822.
The method described above does not require the pilot wafer and, beside, has such an advantage that the positional deviation (offset) can be measured in a short time without effecting the development processing. Further, the following problem is to be produced. When measuring the Positional deviation quantity per shot area on the wafer by utilizing the latent images as described above, a processing time per wafer increases. This results in a decreas in throughput of the apparatus. For this reason, the measurement mentioned above is executed with respect to only, e.g., the top (1st) shot area on the wafer. The reticle pattern may be aligned (alignment) with each of shot areas after the 2nd shot area by use of the positional deviation quantities measured before.
All the on-wafer shot areas are aligned by employing the positional deviation quantity measured in the top shot area. In this instance, however, if the shot area deviates in position due to the expansion/contraction (scaling) or the like of the wafer, the problem is that the precise superposition can not be done even when Giving the above-stated positional deviation quantity as an offset. Moreover, it is difficult to perform the precise superposition even by executing an offset correction based on the positional deviation quantities, depending on a measuring accuracy (reproducibility) of the alignment sensor. Further, the latent images are detected by the alignment sensor using the illumination light exhibiting substantially the same wavelength zone as that of the exposure light. In this case, a problem is caused, wherein optical characteristics of the latent images vary due to the irradiation of the illumination light, and an adverse influence is exerted on the measuring accuracy. In addition, the latent images of the reticle marks are invariably formed in the vicinities of the wafer marks (base marks). A constraint in terms of forming positions thereof is large, and, besides, the latent images are formable as base marks in the vicinities of the wafer marks on the next layer. Consequently, there is also induced such a problem that the alignment accuracy on the next layer is reduced because of the base marks (pseudo alignment marks) of these latent images.
Further in the stepper, it is necessary to effect alignment in X- and Y-directions perpendicular to the optical axis of the aforedescribed projection optical system and alignment (focusing) in Z-direction parallel to the optical axis of the projection optical system. That is, the surface of the wafer must be made coincident with the best imaging plane (best focus plane) of the projection optical system within the range of the depth of focus thereof. For this purpose, the position (best focus position) of the best imaging plane of the projection optical system in Z-direction is found as disclosed, for example, in U.S. Pat. Nos. 4,908,656 and 4,952,815. Then, as disclosed, for example, in U.S. Pat. Nos. 4,558,949 and 4,650,983, the wafer is moved in Z-direction by the use of a focus detecting system (AF sensor) of the oblique incident light type to thereby make the surface thereof with the best focus plane.
However, in the method disclosed in U.S. Pat. No. 4,908,656, the developing process must be carried out, and this leads to the inconvenience that the measurement time becomes long. Also, a pilot wafer exclusively for use for measurement is used, and this has led to a problem that the influence of the ground or the like of a wafer to which the pattern of a reticle is actually transferred (a process wafer) cannot be taken into account and the result of this measurement is not always the best focus position to the process wafer. Further, in the method disclosed in U.S. Pat. No. 4,952,815, the influences of the ground of the process wafer and photoresist or the like have not at all been taken into account and likewise, the best focus position has not always been measured accurately.